Example 43

$\;\;\;(p - 7q)^2$

$=(A - B)^2 \text{, where } A = p \text{ and } B = 7q $

$=A^2 - 2AB + B^2$

$=(p)^2 - 2(p)(7q) + (7q)^2 $

$=p^{2} - 14pq +49q^{2} $